In recent years, the importance of rectifier circuits which operate at low voltages is increasing. For example, radio frequency identification (RFID) tags or noncontact IC cards include no batteries and extract power from radio waves (e.g., 13.56-MHz carrier waves) emitted by readers (readers-writers). In electronic devices, as typified by such RFID tags, which extract power from radio waves and operate, P-N junction (PN) diodes or metal-oxide-semiconductor (MOS) transistors are generally used as rectifier circuits.
In, e.g., a rectifier circuit implemented in a diode-connected MOS transistor, when the voltage (Vgs) between the gate and the source of the transistor is higher than the threshold voltage (Vth) of the transistor, the transistor is turned on and a current flows from the drain to the source. When the voltage between the gate and the source is lower than the threshold voltage of the transistor, the transistor is turned off and no current flows. In the diode, a current flows when the anode voltage becomes greater than the cathode voltage by a forward voltage (Vf) or more.
As described earlier, in the rectifier circuit, a threshold voltage exists at which a switch (a transistor or a diode) is connected and disconnected (turned on/off), so conduction loss occurs upon rectification. The conduction loss may be improved by reducing the threshold voltage.
To reduce the threshold voltage, a shot key diode having a low forward voltage is used as the aforementioned diode, and a MOS transistor having a low threshold voltage is used as the aforementioned MOS transistor. However, the manufacture of, e.g., a MOS transistor having a low threshold voltage results in the addition of another process (manufacturing process), thus raising the manufacturing cost.
By the way, in the past, various proposals have been made for rectifier circuits which operate at low voltages.
Patent Document 1: Japanese Laid-open Patent Publication No. 2001-504676
Patent Document 2: Japanese Laid-open Patent Publication No. H03(1991)-218264
Patent Document 3: Japanese Laid-open Patent Publication No. 2008-085818
Patent Document 4: Japanese Laid-open Patent Publication No. 2006-101670